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1. Field of the Invention
The present invention relates generally to the field of high frequency antennas and more particularly to the field of dual mode, high-gain, planar, high-frequency antennas constructed using inexpensive manufacturing techniques.
2. Discussion of Background
The wireless communication industry""s foremost objective is to provide antennas having (1) the lowest possible manufacturing costs with consistently uniform performance, (2) high gain, and (3) high directivity.
Conventional dipole antennas, in which each member of a pair of quarter wavelength radiators are fed in anti-phase, produce a substantially omni-directional radiation pattern in a plane normal to the axis of the radiators. However, providing such an omni-directional structure on a substantially planar and inexpensive surface, such as a printed circuit substrate, has proven a challenge. Existing attempts to achieve such planarity and performance rely on vias that penetrate the substrate to interconnect a plurality of conducting planes, thereby adding substantially to the cost of the antenna. Extending planar designs over a wide frequency range has proven even more difficult, since many designs only operate over a narrow frequency range.
Improving the gain of omnidirectional antennas is a common goal. Gain improvement is often achieved by designing an array of omnidirectional antennas, stacked on top of each other. Each antenna element must be spaced appropriately and fed with the right amplitude and phase (normally in-phase) to achieve a gain enhancement. Additional gain is realized by narrowing the beamwidth elevation, thereby focusing the same amount of energy into a narrower sector.
In existing designs, as the frequency changes, the phase difference between the two dipoles changes, as result of the feed lines having different lengths. For example, U.S. Pat. No. 6,037,911 discloses a phase array antenna in which the a xe2x80x9cdifferent phase feeding is appliedxe2x80x9d by xe2x80x9cchanging the length of the feeding lines approaching the printed dipoles from outside of the printed patch to the phase center (middle of the antenna).xe2x80x9d
Other designs require the construction of vias through the substrate. U.S. Pat. No. 5,708,446 discloses an antenna that attempts to provide substantially omni-directional radiation pattern in a plane normal to the axis of the radiators. The patent discloses a corner reflector antenna array capable of being driven by a coaxial feed line. The antenna array comprises a right-angle corner reflector having first and second reflecting surfaces. A dielectric substrate is positioned adjacent the first reflective surface and contains a first and second opposing substrate surfaces and a plurality of dipole elements, each of the dipole elements including a first half dipole disposed on the first substrate surface and a second half dipole disposed on the second substrate surface. A twin line interconnection network, disposed on both the first and second substrate surfaces, provides a signal to the plurality of dipole elements. A printed circuit balun is used to connect the center and outer conductors of a coaxial feed line to the segments of the interconnection network disposed on the first and second substrate surfaces, respectively.
However, in order to connect the coaxial cable to the interconnection network, U.S. Pat. No. 5,708,446 requires a via to be constructed through the substrate. This via""s penetration through the substrate requires additional manufacturing steps and, thus, adds substantially to the cost of the antenna.
Furthermore, other attempts require branched feed structures that further increase the number of manufacturing steps and thereby increase the cost of the antenna. A need exists to use fewer parts to assemble the feed so as to reduce labor costs. Present manufacturing processes rely on human skill in the assembly of the feed components. Hence, human error enters the assembly process and quality control must be used to ferret out and minimize such human error. This adds to the cost of the feed. Such human assembled feeds are also inconsistent in performance.
For example, U.S. Pat. No. 6,037,911 discloses a phase array antenna comprising a dielectric substrate, a plurality of dipole means each comprising a first and a second element, said first elements being printed on said front face and pointing in a first direction and said second elements being printed on said back face, and a metal strip means comprising a first line printed on said front face and coupled to said first element and a second line printed on said back face and coupled to said second element. A reflector means is also spaced to and parallel with said back face of said dielectric substrate and a low loss material is located between said reflector means and said back face, whereby said first and second lines respectively comprise a plurality of first and second line portions and said first and second line portions respectively being connected to each other by T-junctions.
However, in order to provide a balanced, omni-directional performance, U.S. Pat. No. 6,037,911 requires a branched feed structure through the utilization of T-junctions. These T-junctions add complexity to the design and, again, increase the cost of the antenna.
Finally, more complex, high frequency antennas have a high loss line structure and, thus, require an expensive dielectric substrate. Due to the simplicity of production and elements and the low cost of the raw materials, the antenna""s cost is significantly lower than for more complicated, high frequency antennas.
Until now, dual mode (aka dual band) antennas have most often been implemented at lower frequencies. Some example dual mode antennas include U.S. Pat. No. 6,198,443, a cell phone dual mode antenna operable in 900 and 1800 MHz bands, U.S. Pat. No. 6,204,826, a dual band antenna disposed on a substrate, and U.S. Pat. No. 4,438,437, a dual mode blade antenna, and others. Some of the previous designs use lumped discrete elements to separate the received bands. At high frequencies, implementing this design (commonly known as a xe2x80x9ctrapxe2x80x9d) becomes difficult due to the deviation of the components from the ideal model, resulting in devices that are impractical to make with reasonable degrees of accuracy and repeatability. The proposed design offers a simple and inexpensive solution to this challenge.
To address the shortcomings of the prior art, the present invention provides several embodiments of a dual mode, substantially planar antenna utilizing monopole, dipole, and dipole array structures for receiving and transmitting high-frequency signals. Opposing layers of conductive strips are disposed on opposite sides of an insulating (dielectric) substrate, such as printed circuit board material.
In one embodiment, a planar two-sided dipole antenna design is extended to operate over two frequency bands by the addition of extra lengths of conductive strips connected to the main dipole elements by inductors. The length of the strips are determined based upon the desired resonant frequencies. At high frequencies, the parasitic capacitance of each inductor provides sufficient capacitance to form an LC notch/trap. This eliminates the need to form external capacitors on the substrate, reducing the cost of the antenna.
In another embodiment, a serial-fed planar high-frequency antenna has multi-dipole elements disposed on opposite sides of a substrate. Each dipole is bifurcated along a horizontal axis, with one half of a dipole disposed on one side of the substrate, and the other half disposed on the opposite side. Each dipole half is in electrical communication with a feed branch independent of the other half. A plurality of dipoles may be dispersed symmetrically along a main feed line. In order to operate over two frequency ranges, additional lengths of conductive strips are attached to each dipole elements via an inductor soldered between the main dipole element, and the extra length of conductive strip. The feed network preferably feeds the dipoles in-phase.
Similarly, a dual-mode parallel feed planar high frequency antenna may be constructed. Opposing layers of conductive strips are disposed on opposite sides of an insulating (dielectric) substrate. Each dipole is bifurcated along a horizontal axis, with one half of a dipole disposed on one side of the substrate and the other half disposed on the other side of the substrate. Each dipole half is in electrical communication with a feed branch independent of its other half. The feed branch on each side of the substrate feeds each dipole half with an equi-distant feed line from a common center point (i.e. feeds each dipole half xe2x80x9cin parallelxe2x80x9d). This provides for a wider operating range, since the dipoles are always fed with the same phase, even as the frequency changes. By connecting an extra conductive strip to each dipole element with an inductor, the antenna is operable over two different frequency ranges.
According to one embodiment of the present invention, the inductors are pre-fabricated inductors that are soldered between the main dipole elements and the extensions. In an alternative embodiment, the inductors may be formed directly on the substrate using a spiral structure. The spiral structure may be, for example, a square xe2x80x9cManhattanxe2x80x9d spiral pattern or a circular spiral pattern. If additional capacitance is required, since a spiral structure inductor has lower parasitic capacitance, a discrete capacitor may be added by forming a plate on each side of the dielectric substrate.
Another type of dual mode antenna proposed is a dual-mode monopole. A monopole is one half of a dipole while the other half is replaced by a ground plane. The ground plane is needed for proper operation of the monopole. The dual mode monopole is fabricated in a fashion similar to that of the dipolexe2x80x94either with a series inductor between the two printed sections or with the printed trap comprising a spiral inductor and a parallel plate capacitor. The monopole may be implemented with either of two types of ground planesxe2x80x94either coplanar with or perpendicular to the monopole.